Thanks for the insight, @erydjunaedy . Is the point that you’re making just that regulations don’t allow you to use a TRANSIENT calculation to compute OTTV and the calculation (for the purposes of regulation) must always be STAEADY-STATE?
If so, I can likely recommend some methods in the new Ladybug Tools SDK that can do a steady state calculation of heat flow across constructions. There will be a lot of inputs for this type of calculation since I don’t know what indoor/outdoor temperatures and incident solar should be used (at least with E+, you know that these come from a TMY EPW or DDY). But it sounds like this would be closer to what you need.